1. Field of the Invention
The present invention relates to a concrete cask suitable for the transportation or long-term storage of radioactive material such as spent nuclear fuels.
2. Description of the Related Art
Concrete casks described in Japanese Patent Applications Laid-open No. 2001-141891 and Japanese Patent No. 3342994 are known as the conventional concrete casks. Japanese Patent Application Laid-open No. 2001-141891 describes a representative conventional concrete cask provided in the top part thereof with a gas outlet opening and in the lower part thereof with a gas inlet opening. In this structure, convection is generated in a gap between the concrete cask and a canister so as to introduce outside air through the inlet opening and release it through the outlet opening. As a result, heat is removed from the canister (sealed container containing the spent fuel) that is stored inside the concrete cask.
Japanese Patent No. 3342994 described a metal cask structure in which a neutron shielding material is provided between an outer shell and an inner shell. In order to enhance the heat transfer between the outer and inner shells, both ends of heat transfer fins made from a metal material with good thermal conduction, such as copper, are connected in their entirety to the inner shell and outer shell. The heat transfer fins are provided radially along the radial direction.
In the structure of Japanese Patent Application Laid-open No. 2001-141891, heat is removed by providing gas inlet and outlet openings and introducing outside air. In this case, corrosion-inducing substances such as sea salt particles contained in the outside air are unavoidably introduced into the concrete cask and adhere to the canister surface. As a result the canister surface is corroded and sometimes stress corrosion cracking can occur under the combined effect with the residual stresses present in the vicinity of welds in the canister. Such cracking means that the containment of canister is disrupted and radioactive material can be emitted to the outside. Furthermore, because the above-mentioned openings serving as the inlet and outlet were the portions that were not covered with a shielding body (portions that lack shielding), radiation streaming from those openings could not be avoided.
In the configuration described in Japanese Patent 3342994, the inner shell and outer shell were connected by both ends of the heat transfer fins in their entirety. Therefore, the problem was that no shielding body was present in the heat transfer fin portions and radiation penetrated through the heat transfer fins and streamed in the radial direction. Furthermore, because of the structure in which the heat transfer fins were in contact with the inner and outer shells, the neutron shielding material such as a concrete had to be placed in the spaces bounded by the inner and outer shells and heat transfer fins one by one, or structural blocks had to be assembled. In this case the manufacture was a time-consuming operation.
It is an object of the present invention to provide a concrete cask that is effective in suppressing the radiation streaming and is easy to manufacture.